This invention relates to a grit blasting machine and method in which particles of metal grit are projected against a surface from which scale is to be removed, with the grit thereafter being recovered and recirculated. In particular, the invention relates to a grit blasting machine and method of the foregoing type in which the grit is recovered by magnetic attraction onto a traveling endless belt positioned within the machine.
In the treatment and preservation of exterior surfaces, such as the decks of ships, and on the highways, etc., it is periodically necessary to clean the surface prior to the application of a suitable coating, e.g., paint. Such cleaning is intended to remove unwanted material on the surface, such as rust, dirt, soot, old paint, and the like, which are collectively referred to hereinafter as "scale". The removal of scale is frequently accomplished by so-called grit blasting machines which project a stream of abrasive metal particles of irregular configuration against the surface at high velocity to detach and remove the scale. To achieve a satisfactory economy in the use of the grit, it has been known to collect the grit and recirculate it. One problem that has become of increasing concern in recent years with the rise in interest in environmental protection measures is to confine the escape of the large quantities of dust that are inevitably generated in the process of grit blasting a scaled surface.
In order to recirculate the grit and minimize the escape of dust, various prior machines have been developed. One such prior machine comprises a vehicle traveling over the surface to be cleaned provided with a housing mounted centrally of the vehicle in which is positioned a throwing wheel for flinging the grit against the surface to be cleaned. A rotating brush following behind the housing and within the interior of the vehicle sweeps up the spent grit and delivers it to a hopper from which conveyors recirculate it to the throwing wheel. A resilient skirt surrounding the lower periphery of the vehicle substantially prevents the escape of dust and grit from the machine and an internal suction system withdraws the dust, subjects it to the action of a dust separator, and collects the dust in a dust collector.
Another type of prior machine, which also includes a traveling vehicle, a throwing wheel, a resilient confining skirt and a dust extraction system, utilizes a rotating magnetic drum to pick up the spent grit. The grit is scraped from the rotating magnetic drum by a blade and falls into a hopper from which it is recirculated to the throwing wheel.
Although generally satisfactory, such prior machines can be subject to certain problems. In both prior machines described, frictional contact is involved in transferring the spent grit to the collecting hopper from which it is recirculated, i.e., either the frictional contact that occurs between the rotating brush and the spent grip particles lying on the surface or the frictional contact that occurs between the scraper blade and the grit particles adhered to the surface of the magnetic drum. Because of the highly abrasive nature of the metallic grit, replacement is inevitably needed in the region where the frictional contact occurs. Such replacement may, for example, be necessitated every four or five hours of operation, thereby leading to unacceptable down time in addition to the direct replacement and maintenance costs involved.
Another problem with the prior machines could occur in conducting cleaning operations on a flat surface (such as a deck) bounded by a vertical wall (e.g., a bulkhead) when there are standpipes or the like adjacent the vertical wall. Because of the standpipes, the vehicle cannot be traversed snug against the wall, but must be spaced from it by at least the width of the standpipe, leaving an unblasted region which must be cleaned by other means. It would be desirable to have a machine in which the blasting head could be shifted laterally in relation to the direction of the travel of the machine. Such a traverse capability would enable the wheel to be positioned closely adjacent the vertical wall and withdrawn laterally to clear a standpipe when necessary.
Other problems of such prior vehicles are associated with the location of the throwing wheel and housing in the center of the vehicle. Because of the central location, it is not possible to position the blasting head really close to a vertical wall because the throwing head is necessarily spaced away from the wall by at least the intervening portions of the vehicle. Moreover, the positioning of the throwing wheel and housing in the center of the vehicle requires complicated maintenance and removal procedures when it is necessary to remove and replace the throwing wheel after it has become worn.